1. Field of the Invention
The present invention relates to a plant control system.
2. Description of the Prior Art
It is desirable from the standpoint of environmental protection that systems for purifying an exhaust gas emitted from internal combustion engines on automobiles, for example, with a catalytic converter such as a three-way catalytic converter and discharging a purified exhaust gas control the air-fuel ratio of an exhaust gas emitted from an internal combustion engine and introduced into the catalytic converter at an appropriate air-fuel ratio which allows the catalytic converter to have a better ability to purify an exhaust gas. The air-fuel ratio of the exhaust gas is more accurately the ratio of air to fuel in an air-fuel mixture which generates the exhaust gas when combusted.
One conventional air-fuel ratio control system combined with an internal combustion engine has been disclosed in Japanese laid-open patent publication No. 5-321721 which corresponds to U.S. Pat. No. 5,426,935.
The disclosed air-fuel ratio control system has an exhaust gas sensor (O.sub.2 sensor) disposed downstream of a catalytic converter for detecting the concentration of oxygen contained in an exhaust gas which has passed through the catalytic converter. The air-fuel ratio control system determines a target air-fuel ratio for the exhaust gas upstream of the catalytic converter according to a PID (proportional plus integral plus derivative) control process such that the oxygen concentration detected by the exhaust g as sensor will be of a predetermined target value. The air-fuel ratio control system then controls the internal combustion engine according to the target air-fuel ratio thereby to place the air-fuel ratio of the exhaust gas introduced into the catalytic converter (the air-fuel ratio of the air-fuel mixture to be combusted by the internal combustion engine) within a given range or window which enables the catalytic converter to have a good purifying ability.
In the above conventional air-fuel ratio control system, the exhaust system, including the catalytic converter, which ranges from a position upstream of the catalytic converter to a position downstream of the catalytic converter may be considered to be a plant for generating and emitting an exhaust gas having an oxygen concentration detected by the exhaust gas sensor, from an exhaust gas having a certain air-fuel ratio. The internal combustion engine may be considered to be an actuator for generating and emitting an exhaust gas having an air-fuel ratio to be supplied to the plant. Thus, the air-fuel ratio control system may be expressed as a system for determining a target input for the plant (more generally, a manipulated variable which defines an input to the plant) such that an output from the exhaust gas sensor (an oxygen concentration of the exhaust gas) as an output from the plant will be equalized to a given target value, and controlling an output of the internal combustion engine (=an input to the plant) as the actuator according to the target input.
As a result of various studies made by the inventors, it has been found that in order to keep the catalytic converter maximally effective to purify the exhaust gas regardless of aging thereof, it is necessary to adjust the concentration of a certain component, e.g., the concentration of oxygen, of the exhaust gas downstream of the catalytic converter to a predetermined target value with high accuracy. In the above conventional air-fuel ratio control system based on the PID control process, it is difficult to adjust the oxygen concentration of the exhaust gas having passed through the catalytic converter highly accurately to a given target value because of disturbances and a dead time present in the exhaust system of the internal combustion engine.
The inventors have devised a control system for controlling the air-fuel ratio of an exhaust gas introduced into a catalytic converter to equalize the output of an exhaust gas sensor positioned downstream of the catalytic converter to a predetermined target value based on a continuous system (specifically, a continues time system) into which an exhaust system including the catalytic converter and ranging from a position upstream of the catalytic converter to a position downstream of the catalytic converter (see, for example, U.S. patent application Ser. No. 08/833,091 (Japanese laid-open patent publication No. 9-324681) and U.S. patent application Ser. No. 08/835,192 (Japanese laid-open patent publication No. 9-273438)).
Based on the above continuous system model, it is possible to compensate for (eliminate) the effect of the dead time present in the exhaust system and construct a control system which employs a control process, e.g., a sliding mode control process for making disturbance control stability higher than a PID control process, and hence for increasing the accuracy and stability of air-fuel ratio control for the exhaust system.
According to the above control system, since the exhaust system including the catalytic converter is modeled as a continuous system, an algorithm for air-fuel ratio control is constructed on the continuous system model. However, because a computer for executing the algorithm can only perform discrete-time processing, it is tedious and time-consuming to effect processing operations with the computer.
Furthermore, inasmuch as the exhaust system including the catalytic converter is modeled as a continuous system, it is difficult to set parameters, including gain coefficients, of the continues system model in a manner to match various operating conditions of the exhaust system, and hence to model the exhaust system with accuracy. If an error between the model and the actual exhaust system, i.e., a modeling error, is too large, then it is difficult to carry out the control process stably for adjusting the output from the exhaust gas sensor downstream of the catalytic converter to the given target value.
The above drawbacks are not limited to the above air-fuel ratio control system, but may also be addressed to any arbitrary system in which in order to control an output from a plant at a predetermined target value, the plant is modeled as a continuous system.